If the acoustic membrane is held fixed between the middle part and the back cover of the watch case, a certain number of side openings may be provided on the periphery of the back cover of the watch for transmitting sound from the vibrating membrane towards the exterior. The acoustic membrane must generally be made of a metallic material of small thickness to ensure good acoustic transmission of the sound or music generated, for example, by means of a musical module of the watch. However, with an acoustic membrane of small thickness arranged between the back cover and the watch movement, the membrane is not able to withstand significant excess pressure or vacuum pressure according to environmental conditions. These environmental conditions concern the physical conditions of the place in which the watch containing the acoustic membrane is situated. High pressure on the membrane can thus occur if the watch is used for a deep dive. In these conditions, the thin acoustic membrane is deformed and comes into contact with parts of the watch movement. This is liable to damage both the membrane and certain parts of the watch movement, which is a drawback.
One solution to this problem is disclosed in CH Patent No 375 664, which discloses a water resistant wristwatch, which includes an acoustic membrane and a means of limiting the bending of said membrane towards the interior of the watch. The circular membrane is fixed to the middle part of the watch, as the back cover, via the peripheral edge thereof. A second back cover of the case is fixed as the external part of the watch on the periphery of the membrane. This second back cover includes side openings for sound to pass through. A stop means is provided for limiting the bending of the membrane towards the interior of the watch. To achieve this, a support member is fixed in a central position on the membrane, and the centre of the second back cover includes a stop member for the membrane support member. The support member passes through an opening in the back cover at the location of the stop member to be fixed to the centre of the membrane. In the event of high external pressure, the membrane support member abuts against the stop member of the back cover to prevent the membrane from bending excessively.
One drawback of the stop means for limiting the bending of the membrane towards the interior of the watch of CH Patent No 375 664 is that the support member is fixed to the membrane by passing through an opening in the second back cover of the watch case. In these conditions, dirt can accumulate in the passage for the support member through the back cover of the case, which may greatly dampen the acoustic transmission resonance from the membrane. This may also disturb the person wearing the watch and possibly cause itching or eczema, which is another drawback. The support member may also be handled from the exterior and the membrane is not removable, which are further drawbacks.